The invention relates to a method of cooling a process stream with an auxiliary stream, wherein the exchange of heat between the process stream and the auxiliary stream is effected in a first heat exchanger and a second heat exchanger connected downstream thereof.
Methods of the generic type for preliminary cooling of a process stream with an auxiliary stream find use, for example, in cryogenic refrigeration systems and liquefaction plants, for example helium and neon refrigeration systems, hydrogen and helium liquefiers, etc. Refrigeration systems and liquefaction plants of this kind generally have a preliminary cooling circuit in which the process stream which is to be cooled and, if appropriate, liquefied is cooled with an auxiliary stream, for example with liquefied nitrogen (LN2). Liquid nitrogen constitutes a comparatively inexpensive refrigeration source. It enables the cooling of the process stream down to a temperature of about 80 K.
The process stream is cooled here with the auxiliary stream in two series-connected heat exchangers. The auxiliary stream or liquefied nitrogen circulated, after it has been refrigeratively expanded, is separated into a liquid fraction and a gas fraction, as elucidated with reference to the FIGURE. While the liquid fraction is conducted in countercurrent to the process stream to be cooled through the two heat exchangers, first through the second, colder heat exchanger, the gas fraction is only conducted in countercurrent to the process stream to be cooled through the first, i.e. the warmer, of the two heat exchangers.
Particle accelerators, fusion research reactors etc. have comparatively large volumes of superconducting magnets and the accompanying installations. These magnets have to be cooled down from ambient temperature (about 300 K) to an operating temperature generally below 5 K. This cooling procedure can take several days and weeks. As already described at the outset, for the first cooling phase from about 300 K to about 80 K, the refrigeration required is preferably provided by inexpensive liquid nitrogen. At the same time, however, the nitrogen must not be conducted directly through the cooling channels of the magnets to be cooled, since nitrogen that remains therein would freeze in the subsequent cooling phases in which cooling is effected down to a temperature of less than 5 K, and block the channels. For this reason, indirect heat exchange between the liquefied nitrogen and the process stream to be cooled is to be implemented.
Owing to their comparatively high efficiency and compact design, preference is given to using countercurrent plate heat exchangers for this purpose. However, these heat exchanger types are sensitive to excessively high temperature gradients between the individual channels and can be damaged or destroyed by excessively high thermal expansion forces.
This risk exists especially during the above-described first cooling phase, in which the process stream to be cooled is cooled down from ambient temperature to a temperature of about 80 K. In the case of conventional cooling and liquefaction circuits, the low- or medium-pressure stream returned from the magnet or experiment to be cooled remains warm for a comparatively long period and is typically returned to the circulation compressor via a warmer at about ambient temperature. In this cooling phase, the high-pressure stream is cooled exclusively in the manner described above by the liquefied nitrogen. The heat of evaporation from the liquefied nitrogen is about the same in terms of size as the difference in enthalpy of the nitrogen through saturated vapor to ambient temperature. In the case of helium refrigeration systems and helium liquefaction plants, the enthalpy profile of helium, by contrast, is constant. Therefore, the temperature spread between the helium process stream to be cooled and the nitrogen stream is at its greatest at the level of the saturated nitrogen vapor in the region between the cold end of the warm heat exchanger and the warm end of the cold heat exchanger.
To date, this problem has been countered by temporarily permitting exceedance of the maximum permissible temperature differential between the channels of the heat exchanger(s). Owing to the risk of damage to the heat exchangers, this reduces the operational safety of the plant. There have also already been proposals to pre-evaporate and heat the liquefied nitrogen to a temperature of at least 50 K below the refrigeration circuit temperature attained—commencing at a temperature of 250 K. However, this procedure is inefficient and comparatively slow.
It is an object of the present invention to specify a method of the generic type for cooling a process stream with an auxiliary stream, in which the above-described disadvantages are avoided.